The printing industry has long recognized that use of water instead of volatile organic solvents, as the volatile component in inks, offers many advantages. In particular, water-based inks are free of the environmental problems commonly associated with solvent-based inks. Further, they exhibit less odor, are less toxic, and are more economical than solvent-based inks. The latter is particularly true when the costs of equipment clean-up of water-based versus solvent-based inks is compared.
Commercial water-based inks typically are composed of water, water miscible organic solvents, natural resins (such as rosin, shellac, dammar or tannin), pigments and a dispersing agent. Water-based inks have been prepared by dispersing the pigment in a vehicle of an acrylic copolymer, acidic rosin ester, shellac or acidic styrene copolymers. (The function of the vehicle is to act as a carrier for the pigment and as a binder to affix the pigment to the printed surface.) See, for instance, U.S. Pat. Nos. 4,908,063 and 5,376,170, herein incorporated by reference.
The performance of water-based inks is dependent on the dispersibility of the pigment in the system. Dispersibility is a measure of the ease with which particulates of pigment can be uniformly and intimately mixed in the system. Poor dispersion of the pigment particulates causes the formation of large agglomerates of pigment particles which, in turn, result in surface imperfections, color streaks, non-uniform coloration and incomplete color development of the resultant ink composition.
Attempts have been made to provide pigment particles with enhanced dispersion and coloring properties for water-based systems. See, for instance, U.S. Pat. No. 4,929,279 which discloses a process of dispersing an organic pigment by the formation of a presscake from a solid surfactant and an aqueous slurry of pigment. Suitable surfactants include a C.sub.18 poly(ethylene oxide) ether having 9 units of ethylene oxide. Such surfactants are further characterized as having a HLB of approximately 12.4. The pigment was dispersed by subjecting the admixture to ultrasonic radiation. Unfortunately, the incompatibility between the pigment and the surfactant caused severe problems. One of these problems was the flocculation of the pigment particulate. The strength of the pigment dispersion was therefore compromised. In addition, the resulting product demonstrates severe foaming problems.
Improved methods for dispersing pigments, which exhibit decreased flocculation, decreased foaming and improved retention of pigment strength in aqueous based systems, especially at low concentrations, is therefore desired.